This invention relates in general to an optical system employing a beam of optical energy for reading a video disc. More particularly, the invention concerns an improved beam control apparatus for maintaining such a beam in tracking and timing registration with the information bearing track of a record disc.
In the field of video information storage and retrieval, the video disc record has been proposed as an adjunct to the conventional home color television receiver to augment the utility of such a receiver by employing it as a playback device for prerecorded video and audio program materials. The program is stored in a medium, such as a vinyl disc which is similar, in some respects, to the familiar audio disc. A record disc of the type herein considered has the video or other information stored in a spiral track which track physically can take the form of a train of pits and lands and is intended to be read by a beam of light, preferably a laser beam. The track is formed under the control of a carrier signal that is frequency modulated with the information to be stored. A disc of the type herein considered is described and claimed in patent No. 3,931,459, which issued to Adrianus Korpel and which is assigned to the assignee of the subject invention.
The stored program material may include luminance, chroma, audio and synchronizing signals, as well as one or more pilot signals, all judiciously positioned in segments of the frequency spectrum deemed to be convenient for recording on a disc but, at the same time, subject to transformation to a frequency distribution that is compatible with the type of signal (NTSC) which the television receiver was designed to process. As such, the information stored in the record track preferably features double interlaced fields for each picture frame, with each convolution of the spiral containing two such fields together with requisite synchronizing information.
The stored information is retrieved by scanning the track with, preferably, a laser reading beam and utilizing a photoreceptor in an arrangement of the type described and claimed in copending application Ser. No. 519,897, which was filed on Nov. 1, 1974 in the names of Robert Adler et al and assigned to the assignee of this invention, now U.S. Pat. No. 4,001,494 which issued Jan. 4, 1977. Adler et al disclose a photoreceptor matrix that responds to the light energy transmitted through the track to develop signals representing stored information, as well as error signals indicative of tracking misregistration and time base error. In the system herein exemplified, as well as in the Adler et al system, the video disc is transmissive to the reading beam, so that the photoreceptor is located along the beam path and in the far field pattern of the diffracted beam on the side of the record opposite the side scanned by the beam. Positioned in this manner the photoreceptor collects a portion of the light transmitted through the disc.
On the other hand, it is appreciated that the disc can be interrogated in the reflective mode, see for example, application Ser. No. 628,717, filed Nov. 4, 1975, in the name of Leonard Laub, also assigned to the assignee of this invention. In the reflective mode situation, the photoreceptor is located on the same side of the track as the reading beam, again in the far field pattern, to enable it to respond to diffracted light reflected from the track. In either the transmissive or reflective modes the pits and lands serve to diffract or scatter the light of the reading beam so that the photoreceptor, in response to these variations in light intensity, develops an electrical signal bearing modulation which is in accordance with the stored information represented by the pits and lands.
In an optical video system, there is no mechanical coupling between the disc and the read beam, thus a system of the type herein considered frequently experiences radial tracking errors due to eccentricity of the disc and/or of the track which results in misregistration of the reading beam and the track. Additionally, time base errors are also encountered when the disc or track is afflicted by eccentricity, as well as when fluctuations in the rotational speed of the disc are experienced. These tracking and timing errors are compensated by means of beam correction apparatus which can take the form of the electro-optical tracking arrangement disclosed and claimed in the above cited Wossidlo patent No. 3,946,166. Wossidlo's arrangement includes a tracking mirror which is inserted in the path of the reading beam and pivotally supported for adjustable displacement about a pair of orthogonally disposed axes coincident with a radius and with a tangent to the track at the point where the track is being interrogated. Respective correction signals, indicative of the sense and extent of radial tracking and time base errors, can be derived in accordance with the teaching of the aforesaid Adler et al patent and applied to assigned drivers which serve to tilt the mirror about its orthogonal axes to restore proper tracking and timing.
However, as noted by Wossidlo, an electro-optical tracking structure of the type he describes exhibits very low mechanical damping and thus has a pronounced mechanical resonance. This is manifested as an unwanted periodic vibration of the tracking mirror which occurs after a mechanical or magnetic disturbance. While this disturbance can be substantially neutralized when the structure is connected into a high gain servo loop, situations present themselves wherein additional damping is desired. To this end, it has been proposed that such damping be provided by introducing a viscous fluid into the pivot bearing that serves to support the adjustable mirror. However, experience has taught that this expedient has pronounced shortcomings.